(1) Field of the Invention
The present disclosure relates to imaging processing of ultrasound diagnostic devices, and in particular to hardness measurement of tissue using shear waves.
(2) Description of the Related Art
In recent years, ultrasound diagnostic devices that have a function of evaluating tissue hardness in a subject have become more prevalent. Methods of evaluating hardness using ultrasound diagnostic devices are broadly divided into two types. One type is a method of evaluating relative hardness of tissue in a subject from magnitude of distortion of the tissue with respect to pressure and a subsequent release of the pressure, the pressure being applied by using an ultrasound probe. This type of method can evaluate relative hardness in a subject, i.e., whether tissue is harder or softer than surrounding tissue.
The other type is a method of generating a shear wave in a region of interest (ROI) in a subject, and measuring propagation speed of the shear wave by acquiring displacement of tissue over a time series in the ROI. Because propagation speed of a shear wave changes according to elastic modulus, this method can evaluate absolute hardness of tissue (for example, elastic modulus). As a method of generating a shear wave, for example, acoustic radiation force impulse (ARFI) is used. Acoustic pressure of an ultrasound push pulse is focused on a focus point, and tissue at the focus point is thereby displaced.